As the development of high-speed digital analog converter (DAC) chips, analog digital converter (ADC) chips and digital signal processing (DSP) chips, the coherent optical communication technology has become a main stream trend of the next generation optical communication. Quadruple spectrum efficiency improvement relative to conventional modulation formats (OOK, on-off keying) may be realized by using an optical in-phase quadrature (I/Q) modulator to map transmission information onto a quadrature phase shift keying (QPSK) constellation and then being polarization-multiplexed.
As all kinds of linear and nonlinear damage exist in an optical fiber link, equalization and compensation need to be performed on a received signal at a receiver side. Although a coherent optical receiver may obtain electrical field information of an optical signal, due to a frequency difference and a phase difference between a local oscillator laser and a transmitting laser, a constant modulus algorithm (CMA) insusceptible to phases is often used as an equalization algorithm at the receiver side.
The basic idea of the CMA is to equalize a received signal by using a linear filter, and coefficients of the linear filer are updated in an iterative manner, the principle of updating being to make difference between the modulus of the equalized signal and a predetermined value be minimum. The CMA may effectively achieve polarization demultiplexing and matching filtering, as well as equalizing residual dispersion and polarization mode dispersion in an optical fiber link. In a case where an updated step is relatively small, the error of an equalized signal is close to a minimum mean-squared error (MMSE). In practical use, for further reducing the complexity, a down-sample manner may be used in updating a CMA coefficient (i.e. an equalizer coefficient), that is, updating is performed every k symbols.
Currently, as requirements on spectrum efficiency are increasing, how to achieve a modulation format with a higher order in an optical communication system becomes a hot spot of interest. For complex modulation formats, such as 16QAM, and 32QAM, etc., an important issue is how to achieve equalization. As these modulation formats satisfy constant modulus conditions no longer, in a case where the CMA is directly used, an optimal linear equalization filter cannot be obtained, even though the coefficients of the CMA are converged.
Various solutions regarding the above issue have been proposed. One of them is a CMA based on multiple moduli, the basic idea thereof being to increase the number of predetermined moduli to n, where, n is equal to the number of different moduli for transmitting signals; for example, in 16QAM, n=3. In calculating an error, the differences between the currently outputted moduli and n predetermined moduli are compared, and a modulus of a minimum difference is selected for subsequent coefficient updating. Another solution is based on decision feedback, wherein after a method of initial convergence is used, when the decided result is deemed as correct modulation information, coefficients of the equalization filter are further adjusted according to the errors before and after the decision.
However, in the implementation of the present invention, the inventors found that drawbacks exist in the prior art: the method of multiple moduli CMA is very complex, and it is not transparent to the transmitted modulation formats, which is because the constellation of the transmission signal decides the sizes and number of the predetermined moduli. And the method based on decision feedback is also very complex, and it is disadvantageous to the parallel realization of the algorithms, for feedback introduced into the whole processing of signals.
Documents advantageous to the understanding of the present invention and the prior art are listed below, which are incorporated herein by reference, as they are fully described herein.
[Document 1]: Irshaad Fatadin, David Ives, and Seb J. Savory, “Blind Equalization and Carrier Phase Recovery in a 16-QAM Optical Coherent System,” Journal Of Lightwave Technology, 3042, VOL. 27, NO. 15, Aug. 1, 2009; and
[Document 2]: Meng Yan, Zhenning Tao, Huijian Zhang Weizhen Yan, Takeshi Hoshida and Jens C. Rasmussen, “Adaptive Blind Equalization of Optical BPSK system,” Th9A4, ECOC, 2010.